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Box fill calculator

Count the conductors, grounds, clamps, devices and support fittings in an outlet or junction box to get the minimum box volume in cubic inches under NEC 314.16(B), and see which standard metal boxes from Table 314.16(A) are big enough.

Wire
AWG

Every allowance uses this size (Table 314.16(B)(1)).

Three 12/2 cables = 6. Pigtails that stay in the box count 0.

Up to four count as 1; each extra adds 1/4.

In the box

Each yoke counts as 2.

Any number of internal clamps counts as 1. External connectors count 0.

Advanced options
Fittings

Each counts as 1.

Required box volume

22.5 in³

10 allowances × 2.25 in³ (12 AWG)

Volume allowances
10.00
6 conductors + 1 for 3 grounds + 1 for clamps + 2 for 1 device + 0 for fittings
Volume per allowance
2.25 in³
Table 314.16(B)(1), 12 AWG
Square box that fits
4 × 2-1/8 in
30.3 in³ · up to 13 × 12 AWG with nothing else in it
Single device box that fits
None
No single-gang metal device box in Table 314.16(A) is big enough.
Metal boxes that fit
4
4 × 2-1/8 square 30.3 · 4-11/16 × 1-1/4 square 25.5 · 4-11/16 × 1-1/2 square 29.5 · 4-11/16 × 2-1/8 square 42.0
  • No single-gang device box is large enough; use a 4 in square box with a mud ring or a multi-gang box.
  • Plastic boxes: compare 22.5 in³ with the volume stamped inside the box.
Box fill: 10 allowances × 2.25 in³ = 22.5 in³22.5 in³ required against a 4 × 2-1/8 square box of 30.3 in³, 7.8 in³ spare.Allowances stacked to scale inside the box volume4 × 2-1/8 square · 30.3 in³Conductors 6 × 2.25 = 13.5 in³Grounds 1 × 2.25 = 2.25 in³Clamps 1 × 2.25 = 2.25 in³Devices 2 × 2.25 = 4.5 in³22.5 in³014 × 2-1/8 square30.3 in³ box0222.5 in³ = 74% full7.8 in³ spare03
Default example: three 12 AWG cables with grounds, internal clamps and one device: 10 allowances × 2.25 in³ = 22.5 in³, stacked inside a 4 × 2-1/8 square box. Heights are to scale by volume.
  1. 01 Required volume: the allowances stacked to scale
  2. 02 Square box compared (smallest that fits, or the largest if none does)
  3. 03 Spare volume left in that box (or the overfill)

How box fill is calculated

Box fill counts volume allowances. Each item in the box is worth a number of allowances, and each allowance needs the cubic inches that Table 314.16(B)(1) lists for the conductor size. This calculator prices every allowance at the largest conductor in the box.

ground allowance = 0 with no grounds, otherwise 1 + 1/4 × (grounds beyond 4)

allowances = conductors + ground allowance + 1 (if internal clamps) + 2 × devices + 1 × support fittings

required volume = allowances × in³ per conductor

Any metal box from Table 314.16(A) whose volume is at least the required volume is big enough. The calculator lists the round, square and single device boxes that fit. For the square box it picks the shallowest 4 in square that fits, then the 4-11/16 in sizes; for the device box it picks the 3 × 2 in boxes first, then the 4 × 2-1/8 in utility boxes.

Volume per conductor, NEC 2023 Table 314.16(B)(1)
Conductorin³ each
14 AWG2.00
12 AWG2.25
10 AWG2.50
8 AWG3.00
6 AWG5.00

Worked example: three 12/2 cables and a receptacle

The calculator opens on a common receptacle box: three 12/2 cables, so 6 insulated conductors and 3 grounds, internal cable clamps and 1 receptacle.

  1. Conductors: 6 allowances.
  2. Grounds: 3 grounds is 4 or fewer, so 1 allowance.
  3. Clamps: 1 allowance. Device: 1 yoke × 2 = 2 allowances.
  4. Total: 6 + 1 + 1 + 2 = 10 allowances.
  5. Volume: 10 × 2.25 in³ = 22.5 in³.

The deepest single device box in the table, the 3 × 2 × 3-1/2 device at 18.0 in³, is too small. The smallest square box that fits is the 4 × 2-1/8 square at 30.3 in³, used with a single-device mud ring. 4 metal boxes from the table are large enough:

Table 314.16(A) boxes that hold the default example (22.5 in³, 12 AWG). This table is fixed to the default example; the results panel lists the boxes for your own inputs.
Box (in)Volume, in³Max 12 AWG aloneSpare, in³
4 × 2-1/8 square30.3137.8
4-11/16 × 1-1/4 square25.5113.0
4-11/16 × 1-1/2 square29.5137.0
4-11/16 × 2-1/8 square42.01819.5

"Max alone" is the Table 314.16(A) conductor limit with nothing else in the box: volume ÷ 2.25 in³, rounded down.

What counts and what doesn't

  • Count 1 for every insulated conductor that comes in from outside the box and is spliced or terminated, and for one that passes straight through. A loop of at least twice the 300.14 free length counts 2; enter it as two conductors.
  • Count 0 for pigtails and jumpers that never leave the box, for wire connectors, and for external cable connectors.
  • Grounds: all of them together count 1, plus 1/4 for each beyond 4. 6 grounds count 1.5. This 1/4 rule dates from the 2020 NEC.
  • Devices: 2 for each yoke. A device wider than a single 2 in gang counts 2 for each gang it needs.
  • Support fittings: 1 for each fixture stud or hickey.

Isolated-ground conductors for isolated-ground receptacles are counted with the other grounding conductors under 314.16(B)(5); the separate extra allowance they once had was removed in the 2020 NEC. Include them in the ground count. A terminal block counts one allowance under 314.16(B)(6), new in 2023; the calculator does not model terminal blocks, so add one conductor for each.

Plastic, ganged and extended boxes

Plastic boxes are not listed in Table 314.16(A). Check the cubic-inch rating molded or stamped inside the box, and buy one at or above the required volume. Masonry boxes and FS and FD cast boxes are in the table, but the calculator leaves them out of the fit list. Compare their volumes by hand.

Ganged boxes add their volumes. Two 3 × 2 × 3-1/2 device boxes ganged together give twice the volume of one. Plaster rings, extension rings and domed covers add their marked volume too, which is often the easiest way to gain room in a crowded square box.

Sources

  • NFPA 70, National Electrical Code, 2023 edition, Section 314.16(B)(1)–(6) (counting conductors, clamps, support fittings, devices, grounding conductors and terminal blocks)
  • NFPA 70, 2023, Table 314.16(B)(1) (volume allowance required per conductor; numbered Table 314.16(B) before 2023)
  • NFPA 70, 2023, Table 314.16(A) (metal boxes) and 314.16(A)(2) (volume marked on other boxes)

Frequently asked questions

How many cubic inches does a 12 AWG conductor need?

Table 314.16(B)(1) gives 2.25 in³ for each 12 AWG conductor, 2.00 in³ for 14 AWG and 2.50 in³ for 10 AWG. Multiply the total number of allowances in the box by the value for the largest conductor to get a conservative box size.

How do devices and grounds count toward box fill?

Each switch or receptacle yoke counts as 2 conductors, based on the largest conductor connected to it. All the equipment grounding conductors together count as 1, plus 1/4 for each one beyond the first 4.

Do cable clamps count in box fill?

Internal cable clamps count as 1 allowance in total, no matter how many there are. External clamps and connectors whose body sits outside the box count zero.

How many cubic inches is a 4 in square box?

Table 314.16(A) lists a 4 × 1-1/2 in square box at 21.0 in³ and a 4 × 2-1/8 in square box at 30.3 in³. A raised cover or extension ring adds the volume marked on it.

Do pigtails count toward box fill?

No. A conductor that starts and ends inside the box, such as a pigtail or a jumper between devices, is not counted. Wire connectors are not counted either.

How do I size a plastic box?

Nonmetallic boxes are not in Table 314.16(A). Use the volume molded or stamped inside the box and compare it with the required volume from this calculator.

What if the wires in the box are different sizes?

The code counts each conductor at its own size and bases clamp, fitting and device allowances on the largest conductor involved. This calculator uses the largest size for everything, which gives the same or a slightly larger box.

By Tradesman Calculator editorial team · Formulas reviewed · How we check accuracy · Report an error